Excess sludge generated from the installations for purifying sewage water from a sewage works or a domestic septic tank, by biological treatment, is first dewatered and usually used for landfill or incinerated as industrial wastes. Recently, it has become necessary to introduce a large-scale drying device, in order to combat the problem of shortage of landfill site or of the probability of dioxin generation ascribable to the lowering of the temperature of the combusting furnace at the time of the incineration, thus posing the problem of high cost.
For biologically reducing the volume of excess sludge, there has so far been known an aerobic or anaerobic method consisting in employing aerobic or anaerobic microorganisms. However, the method of anaerobic digestion has a defect that the digestion time is prolonged and the digestion rate is low and hence the method is currently not used extensively.
There is disclosed in Patent Publication 1 a method for reducing the volume of sludge by first dissolving sludge by alkali addition and heating and subsequently subjecting the resulting mass to an aerobic biological treatment method. There is also disclosed in Patent Publication 2 a method for reducing the volume of sludge by heating and pressurizing sludge, solubilizing the resulting mass on pulverization and subjecting the resulting mass to aerobic biological treatment. There is furthermore disclosed in Patent Publication 3 a method for reducing the volume of sludge by solubilizing the sludge under subcritical conditions and subsequently subjecting the resulting mass to aerobic biological treatment. In any of these methods, it is necessary to completely solubilize and lower the molecular weight of the cell wall of sludge, as a substance difficultly degradable by microorganisms, if the solubilized sludge is to be metabolically decomposed by the aerobic biological treatment method. Notwithstanding, the sludge is not solubilized sufficiently with the method disclosed in Patent Publication 1 or 2, with the consequence that the sludge can be reduced in volume only slightly. On the other hand, with the method disclosed in Patent Publication 3, since the biodegradation-resistant substances are yielded by the vapor phase reaction under an elevated temperature, the solubilized sludge is poor in metabolic degradation performance, with the result that the time needed for metabolic degradation is protracted, thus raising the cost.                [Patent Publication 1] JP Patent Kokoku JP-B-49-11813        [Patent Publication 2] JP Patent Kokai JP-A-2000-354896        [Patent Publication 3] JP Patent Kokai JP-A-2000-218285        